The United States Environmental Protection Agency (EPA)'s mission is to protect human health and the environment. To achieve this mission, EPA implements a variety of programs under the Clean Air Act that focus on reducing outdoor, or ambient, concentrations of air pollutants that cause smog, haze, acid rain, and other problems; reducing emissions of toxic air pollutants that are known to, or are suspected of, causing cancer or other serious health effects; and phasing out production and use of technologies that destroy stratospheric ozone. These pollutants come from stationary sources (such as chemical plants, gas stations, and power plants) and mobile sources (such as cars, trucks, and planes). State and local governments have also enacted legislation and implemented guidelines to encourage the development of clean technologies.